This invention provides a relatively inexpensive and convenient tool for rapidly measuring the headspace of cartridge cases, this being especially useful for gunsmiths and those that reload cartridge cases.
Useful background for an understanding of this subject is the article entitled "HEADSPACE . . . AND HOW IT IS MEASURED" by Major General J. S. Hatcher that is found on pages 67-69 of the book entitled "HAND LOADING" copyright 1981 by the National Rifle Association of America.
Rimmed cartridges, either rim fire or center fire, have a head which is larger in diameter than the body of the case. However, because rimmed cartridges present difficulties in feeding from box magazines, both center fire automatic pistols and many late model center fire rifles use rimless cartridges. The head of tie tireless cartridge is normally no larger in diameter than the body of tie case, but it does have a groove for the extractor cut into the body of the case just forward of the case head. As the rim thus formed is no larger than the body of the case forward of the head, there is no convenient way of using the rim for slopping the cartridge in chambering, and the cartridge stops by having its conical shoulder come in contact with a corresponding shoulder in the chamber.
For such cartridges, headspace is measured from the face of the closed bolt of the fire arm to a point where a circle of a certain size, called a datum circle, would intersect the cone of the shoulder on the chamber. The Hatcher publication at FIG. 9 depicts a number of prior art headspace gauges.
Headspace can either be too small or too great. Variation of headspace of either sense can create significant problems. For example if the chamber headspace is not large enough the bolt will not then close completely on a rifle, automatic pistol, or shotgun cartridge.
the cartridge headspace may be found too great in some cases because the cartridge has been stretched (in the case of reloads) by firing it in a gun that had excess headspace. If the casing is then reloaded without full length re-sizing and an attempt is made to use it in a gun with tight headspace, it may be found impossible to close the bolt.
If there is excess headspace in a gun there may be ignition trouble (misfires or hang fires) because the firing pin may not be able to give the primer a proper blow. Also accuracy may be diminished with excess headspace.
When a gun such as a high power rifle with excess headspace does fire, a stretched or ruptured cartridge may result with possible dangerous escape of gas to the rear around the bolt head. This rupturing of the cartridge case occurs because the firing pin blow drives the cartridge forward and then the internal pressure of the ignited powder expands the neck and sidewalls of the case outward and causes them to grip the walls of the chamber. The friction of the case walls against the chamber, together with the forward pressure of the gas on the inside of the case shoulder, combine to hold the front portion of the case forward, while the pressure of the powder gas acting on the inside rear of the case blows it back, causing a stretching, or even a separation, between the front and rear portions of the cartridge case.
Thus those skilled in the art know the importance of the measurement of headspace on a cartridge prior to the reloading thereof.
Another prior art headspace gauge is shown in the L. E. Wilson U.S. Pat. No. 3,209,461 issued Oct. 5, 1965.